Because if you want the reply to go to the right topic that it’s in reply to, that key is necessary.
What mail delivery service are you using? It’s odd that it would not deliver mail with a +XXX address.
It’s not Discourse’s fault that your mail transfer agent silently throws away mail if it doesn’t like the sender address. It can’t know that your mailing will throw them away. The logs that you seek are at your mail delivery service, not Discourse.